The present invention relates generally to computer-based process control systems and more particularly to an apparatus for reducing heat dissipation by a plurality of high voltage contact closure inputs in a process control system.
Many industrial process control systems employ computers as an efficient means for controlling a complex operation. One method by which a process control computer is informed of the status of a given process is by monitoring the state of relay contacts. Relays, or any other devices which may open or close contacts in response to a stimulus from a process under control, are positioned with a process apparatus so as to indicate the status of certain elements of the process apparatus, such as motors or switches. To provide status information to the computer, a current loop is established from a power supply, through the relay contacts, through a contact closure input module and back to the power supply. When the relay contacts are closed, current flows through the loop and the input module provides a signal to the computer indicating the state of the contacts. Relay contacts may be either normally open or normally closed, as control system specifications demand. It is not uncommon for a large process control system to involve monitoring the state of several thousand relay contacts.
The contact closure input modules are typically current sensitive devices. When the connected relay contacts are closed, the full voltage of the current loop power supply is applied across the input module. Each input module generally contains a dropping resistor in series with a light-emitting diode (LED) of an optoisolator. When current flows through the module, the dropping resistor dissipates most of the power supply voltage. Voltage not dissipated by the dropping resistor activates the LED, ultimately signalling to the computer that a relay has been operated. Where the current loop power supply is a relatively high voltage supply, such as 125 volts DC, the heat generated by the dropping resistor can be substantial.
The present trend toward increasing the complexity of process control systems is parallelled by efforts to increase the efficiency and thereby decrease the cost of existing process control techniques. One method of increasing system efficiency is to redesign control system architecture to reduce the number of cables concentrated at the process control computer. By establishing remote data collection cabinets to house the input modules for a plurality of relay contacts located within a given area, the contact status of many contact relays may be communicated to the process control computer over a single coaxial cable or pair of wires.
The remote data collection cabinets are located as close as possible to the relay contacts connected to that cabinet. This often requires that the data collection cabinets be placed in extremely hostile electrical and thermal environments. Large electromagnetic fields, wide variations in ambient temperature and dusty working conditions mandate use of a dust-tight, electrically conductive enclosure for the data collection cabinet. While providing an effective defense against a harsh environment, the sealed enclosures do not permit dissipation of heat generated by electronic circuits within the enclosure. If the circuits located within the enclosure generate too much heat, the rate of failure of the enclosed circuits increases and the cost effectiveness of the data collection cabinets is defeated.
The problem of heat dissipation is compounded when system specifications necessitate use of a high voltage power supply, such as 125 volts DC, to sense contact status. Existing input modules utilize optoisolators to convert the contact closure input signal, typically a direct current of 10 to 15 milliamperes, to a signal recognized by the process control computer. The energized LED of the optoisolator activates a phototransistor which causes a switching action in logical devices connected to the computer. Hence, the contact closure input module converts the direct current input signal to a DC voltage signal at the computer. The dropping resistor within the input module is used to reduce the 125 volt DC input signal to approximately 1.7 volt DC, which is the forward bias voltage across the LED of the optoisolator. As a result, the dropping resistor may dissipate as much as 1.85 watts in each input module. A plurality of input modules, each utilizing a contact-sensing voltage of 125 volts DC, may easily give rise to excessive temperatures within a sealed data collection cabinet.
It would therefore be advantageous to develop an improved scheme for sensing the status of high voltage relay contacts within a sealed data collection cabinet which can be located in hostile process environments near the relays. Such a scheme should reduce the heat dissipated by a contact closure input module when the connected relay contacts are closed, so as to permit collection of a plurality of such modules in the sealed enclosure without danger of overheating.